Carbon-fiber reinforced plastic composites (CFRP) have seen increasing use as structural components in aircraft due to their high specific stiffness and strength. One issue with the use of CFRP is that thermal degradation of the matrix can lead to significant decreases in the glass transition temperature, mechanical properties such as flexural strength, compression after impact (CAI), and interlaminar shear strength, and cause delaminations, fiber-matrix debonding, and embrittlement and cracking of the matrix. For aircraft this thermal damage can come from sources such as fires, lightning strikes, ground-reflected efflux from the engines, accidents, etc. Of even greater concern is that below a certain threshold exposure level, the part may appear visibly undamaged and it can also appear undamaged to common nondestructive evaluation (NDE) methods such as ultrasound techniques, but the part can exhibit up to 60% loss of strength. This type of damage is often termed incipient thermal damage.
Many different techniques have been utilized to try to evaluate and detect incipient thermal damage to CFRP parts including FTIR, laser-induced fluorescence (LIF), Raman spectroscopy, and NMR. While many of the techniques have been shown capable of detecting incipient thermal damage most of them are not viable options for inspection of parts in service. Currently the most prominent means of detecting incipient thermal damage for in service inspection is diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). DRIFT spectroscopy is capable of detecting and providing quantitative information changes to the functional groups of the matrix which are affected as the matrix thermally degrades. Changes in the carbonyl and phenol bands of the FTIR spectrum were found to correlate fairly well with changes to mechanical properties such as ILSS and were sensitive to early signs of thermal oxidation before significant strength loss occurred. One of the main issues with DRIFT spectroscopy though is that it has a very small effective inspection area relative to the size of many aerospace parts so it is not a very efficient wide-area technique. As a result it can be difficult to locate and evaluate thermal damage sites on large CFRP parts if the damage site is not already known. Another method that showed promise as a wide-area inspection technique for incipient thermal damage was laser-induced fluorescence (LIF). LIF works by using a laser excitation source to excite the autofluorescence of the matrix. It has been found that both the intensity and the wavelength at the max intensity λmax, change as a result of thermal damage, however only the λmax was shown to correlate directly to changes in mechanical properties such as flexural strength. Both DRIFT and LIF are only surface sensitive techniques however, so considerable testing needs to be done to determine how well they apply to bulk materials. Thermo-elastic characterization has been shown as possible method for detecting volumetric thermal degradation. Thermo-elastic characterization works by using an acoustic horn to generate a high amplitude acoustic wave that generates heat as it passes through the material and an IR camera is used to measure the change in temperature. Potential thermal damage can be observed by changes in the slope of temperature over the amplitude of the wave (ΔT/ΔA). Thermo-elastic characterization has also shown good potential for finding incipient thermal damage however, there is very little literature available so it is difficult to evaluate its applicability to in field use.
A common issue with all of these inspection methods (except for LIF) is that while they are capable of detecting thermal damage on a sample, they are not very efficient for inspecting large parts if the damage location is not known. Because incipient thermal damage of CFRP can be very difficult to locate visually, this can be a problem for inspecting large aircraft parts for thermal damage quickly. Therefore, improved inspection methods for incipient thermal damage of CFRP are desired.